I wrote:
> 1) Does this really pass muster from the translatability standpoint?
> I doubt it.
After further thought about that, it seems that what we typically don't
try to translate is SQL-standard type names, that is, error messages
along the line of "blah blah blah type %s" are considered fine. So
the problem here is that you factorized the error reporting poorly.
I think you want the callers to look like
if (!JsonbExtractScalar(&in->root, &v) || v.type != jbvNumeric)
cannotCastJsonbValue(v.type, "double precision");
where the subroutine contains the whole ereport() call, and its lookup
table entries are e.g.
gettext_noop("cannot cast jsonb string to type %s")
regards, tom lane
